Watermarks have been used for centuries to mark and to recognize documents having a specific content or value. Watermarks are formed during the production of paper inter alia by influencing in a targeted manner the dewatering of a fibrous suspension from which the paper is made. For this purpose, a dewatering screen was originally provided with small, water-impermeable elements, as a result of which during the forming of sheets the deposition of fibres on the dewatering screen at the position of these elements differs from the deposition of fibres at locations of the screen without elements of this type. The effect is a light feature which is clearly perceptible when looking through. Later technological developments have led to what is known as the shadow watermark, a watermark with the most nuances in terms of shades of grey.
Shadow watermarks of this type are usually formed during the sheet-forming process with the aid of a round sieve as the dewatering screen. Elevated and lowered parts are introduced in the dewatering screen, which is usually made up of a plurality of layers of gauzed material, at least in the outermost layer of the screen. A homogeneous fibrous suspension is then brought into contact with the surface of the screen and sheets are formed on this surface, as a consequence of dewatering. The ultimate inhomogeneities in the distribution of fibres over the sheet are the result of deliberately introduced non-uniformities in or on the screen. More fibrous material is deposited in lowered parts of the screen than on elevated parts of the screen; the degree of elevation and lowering results ultimately in a specific greyscale value when looking through. Even fewer fibres are deposited in areas where larger (the term “larger” being used in this case in relation to the length of the fibres) completely water-impermeable parts are present than in the elevated parts of the screen. When the paper is eventually dry, then the parts containing the most fibrous material will be darker when looking through than the average and the parts containing the fewest fibres will be lighter. The thinnest parts in the paper are produced at the site of the completely water-impermeable parts. Because a shadow watermark of this type comprises both elevated and lowered parts and all transitions therebetween, many shades of grey can be produced, as a result of which shadow watermarks are obtained. The watermarks, use being made of only lighter parts with respect to the planum (=average greyscale value), will by definition have a lower greyscale value tonality than a complete shadow watermark.
The making of a dewatering screen, such as a round sieve with watermark elements, demands much experience and expert knowledge and is, moreover, very labour-intensive and as a result expensive. In a round sieve for the production of banknotes, several hundred watermark regions are often present on or in the layers of the screen. All these regions must ultimately yield an identical end result in the banknotes which are produced, so that very stringent demands are placed on the dimensioning and reproducibility.
A traditionally constructed round sieve with shadow watermark regions often consists of at least two gauze layers made of metal wire, referred to hereinafter also as the wire screen, the bottom layer(s) serving as supports for the outermost layer. The entity as a whole is vulnerable to mechanical damage. Undesirable disturbance, such as damage, in the surface of the screen causes a correspondingly undesirable effect in the forming of sheets. Undesirable disturbance in the watermark regions of the screen is usually immediately problematic. Mechanical damage occurs, during the production of paper, especially in the elevated parts of the surface of the screen. In the long run, this renders each dewatering screen with a watermark region unusable; a new, very expensive screen or outermost screen layer (screen cover) will then have to be made.
It is evident that the (limited) lifetime of a dewatering screen is one of the factors influencing the total production costs of paper with a watermark. Efforts have therefore been made to minimize the mechanical loading of the surface of the screen by transferring the sheet, as soon as it has been formed, from the dewatering screen to a take-off screen or cloth via a reduced pressure transfer technology; in this case, there is, generally speaking, no longer any mechanical contact between the couch roll and the surface of the screen and, as a result, the surface of the screen is spared.
Recently, DE 102005042344 has proposed locally providing a screen cover with a plate-shaped, perforated material which is brought, together with the screen lining, into a common relief form. The perforations in the plate-shaped material have micro dimensions. The advantage of this modified screen lining is said to reside in the creating of shadow watermarks provided with very light parts, also called electrotypes.
DE 10064006 explains how those parts of a watermark region of a dewatering screen that ensure very light parts in a watermark image, the so called electrotypes, can be fitted as an additional element in a watermark region with the aid of a shape memory material.
DE 10 2006 058 513 and WO 2008 071325 describe a watermark region consisting of an injection-moulded plastics material element with a relief, which relief is provided with perforations with the aid of a laser. For this purpose, holes are formed in a profiled element from the side opposing the profile using the laser. In an embodiment, the perforations become narrower from the back (dewatering side) in the direction of the relief side. It is stated that the perforations ensure free flowing of the paper suspension and that the watermark element regions having a greater material thickness allow the formation of thin spots in the paper.
A drawback of this known method is the fact that the precision of the perforation, in particular the diameter at the relief side, is highly dependent on the material properties, such as the nature of the plastics material and the local material thickness, and as a result the precision is difficult to control. The action of the laser produces holes which narrow toward the profile side. As a result of the physical processes which are involved in the perforating of a plastics material using a CO2 laser, the shape of a perforation is, in the longitudinal direction of the channel, more or less conical at the lasered side owing to the Gaussian energy distribution of the laser beam and the longer the channel is, the more this conical shape decreases. This provides narrowing perforations, the diameters of which at the profile side are in some way related to their height position in the profile. The decrease of the conical shape prevents the differences in greyscale value from being controlled more precisely in the final watermark image. The use of the laser for precisely controlling the diameter of the perforation as a function of the height of the profile thus leaves much to be desired.